I found an unusual slug in the grass verge of my vegie garden which doesn't
get any sprays. It was about five cm long, about 1cm wide, quite flat and
was black with a cobalt blue stripe down one side. The site is 20 metres
above the sea and well away from the native bush line.
What sort of slug could this be?

Maria Minor, an ecologist at Massey University's Institute of Agriculture
and Environment, responded.

From your description, it sounds like the animal you found was not a slug
at all, but a terrestrial flatworm (Turbellaria). They are quite flat, and often
have a longitudinal stripe on their back. Flatworm come in different colours,
even pink, blue, and yellow, but many are just dark with a stripe.

Pictures of terrestrial flatworms can be seen at the SoilBugs website
(soilbugs.massey.ac.nz). These are different species from what you
found, so will not look exactly the same, but the pictures give the idea
of their general body shape.

Terrestrial flatworms are predators, and feed on earthworms and other
invertebrates. Flatworms usually hide during the day, but come out at
night, and hunt by following the scent of their prey. The flatworm you
found is probably native - there are more than 200 species in New Zealand.
Flatworms are common and widespread in New Zealand, but not very
well studied, and many remain unknown to science.